(a) Field of the Invention
The present invention relates to a method for compensating a hydraulic pressure of an automatic transmission. More particularly, the present invention relates to a method for compensating a hydraulic pressure of an automatic transmission that reduces shift shock and increase shifting speed.
(b) Description of the Related Art
When a brake is operated while a lift-foot-up shift is performed, a time required for a turbine speed of an automatic transmission to reach a synchronization speed is increased. Therefore, a shifting time is increased.
In this case, a shift shock may increase, and a delay may occur in response to a re-acceleration.
FIG. 4 illustrates a conventional scheme of a method for compensating a hydraulic pressure of an automatic transmission.
Several terms relating to shifting control is explained below
When it is determined that a shift is required on the basis of a vehicle speed and throttle valve opening, a transmission control unit (TCU) starts a required shift control by starting control of a solenoid valve in the AT, which is usually called “shift-start point” and is abbreviated as “SS point”.
By starting of the solenoid valve control, after a certain period, an off-going friction member begins releasing its hydraulic pressure, and an on-coming element begins to be supplied with a hydraulic pressure, which is usually called “shift-begin point” and is abbreviated as “SB point”. The period after the SS point to the SB point becomes a delay period which is not used for an actual shifting operation of the automatic transmission (“AT”).
So, an actual shifting period (also called an inertia phase) of the AT begins at the SB point and finishes at a time point at which the off-going element is fully disengaged and the on-coming element is fully engaged. Such a time point at which the off-going element is fully disengaged and the on-coming element is fully engaged is usually called “shift-finish point” and is abbreviated as “SF point”.
As shown in FIG. 4, an opening of a throttle valve Th is zero percent (0%) at the shift-start (SS) point, since the accelerator pedal is in a lift-foot-up state. At this time if a brake is operated, the shifting operation is performed as follows.
A turbine speed Nt gradually decrease from the shift-start point SS, and it is abruptly synchronized to a synchronization speed near the shift-finish point SF1.
An output shaft rotation speed No decreases after when a brake is operated, since the vehicle speed decreases by the braking operation. Then, a conventional on-coming hydraulic pressure Pr that is supplied to an on-coming friction member (e.g., a clutch or a brake) in the automatic transmission linearly increases with a first-order slope hydraulic pressure Pg for a predetermined period before the shift-finish point SF1. The on-coming hydraulic pressure Pr is controlled to abruptly increase at the shift-finish point SF1.
However, according to the above conventional shifting operation, the time for the turbine speed Nt to reach the synchronization speed Ns is long, since the gradient of the conventional on-coming pressure Pr applied to the friction member is applied the same till the shift-finish point SF1. In addition, a large shift shock may occur since the turbine speed Nt may be abruptly changed to the synchronization speed Ns at the shift-finish point SF1.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.